Techniques are known for controlling emissions of sulfur dioxide (SO.sub.2) or nitrogen oxides (NO.sub.x) which are toxic oxidation products emitted from combustion systems such as power plants.
Wet scrubbing towers permit effluent gases to pass through beds of limestone, dolomite, and other calcium-containing compounds or catalysts. U.S. Pat. Nos. 3,962,864; 4,178,357; 4,301,425; 4,304,550; 4,313,742; and 4,562,053 illustrate various devices for cleaning flue gases. Wet scrubbing devices tend to be expensive because their complexity requires high operating costs. They also suffer from severe corrosion and plugging problems.
The wet scrubbing process has constantly been modified in the attempt to down-size the scale of equipment. U.S. Pat. No. 4,861,577 describes a method wherein exhaust gas is absorbed into a scrubbing solution which is then treated in an autoclave to decompose such compounds as thiosulfate and polythionates into elemental sulfur and sulfate. U.S. Pat. No. 5,019,361 discloses an amine salt absorbent that purportedly permits high recovery of sulfur dioxide with smaller equipment.
A technique related to wet scrubbing is the spraying of water slurries or dry powders of sorbents. Various spraying devices, some of which are used in conjunction with wet scrubbers, are shown in U.S. Pat. Nos. 4,001,384; 4,323,371; 4,419,331; and 4,530,822. These facilitate gas/liquid contact by atomization of liquids into flue or stack gases. Injection of sorbents can be implemented in the furnace or post-furnace zone, depending upon the thermodynamic and kinetic processes involved, thereby increasing the flexibility of the spraying technique.
Sorbent particles, ideally, should be small in size, porous, and able to mix well with the gases that are to be cleaned of pollutants. Typical sorbents are listed below ("Alternative SO.sub.2 Sorbents," PSI Technology Company Report PSI-538/TR-744, 1987. RESEARCH PARK. P.O. BOX 3100, Andover, Mass. 01810):
______________________________________ Sorbent Class Sorbent Type Formula ______________________________________ Lime/Limestone Hydrated Dolomite Ca(OH).sub.2.Mg(OH).sub.2 Hydrated Lime Ca(OH).sub.2 Limestone CaCO.sub.3 Dolomite CaCO.sub.3.MgCO.sub.3 Alkali Trona Na.sub.2 CO.sub.3.NaHCO.sub.3.2H.sub.2 O Nahcolite NaHCO.sub.3 Mixied Cation Shortite Na.sub.2 Ca.sub.2 (CO.sub.3).sub.3 Gaylussite Na.sub.2 Ca(CO.sub.3).sub.2.5H.sub.2 O Pirssonite Na.sub.2 Ca(CO.sub.3).sub.2.H.sub.2 O Eitelite Na.sub.2 Mg(CO.sub.3).sub.2 ______________________________________
Upon injection into high-temperature environments, sorbents containing calcium undergo calcination or decomposition to an oxide (CaO). The same holds true for magnesium-based sorbents which oxidize to MgO, as taught in U.S. Pat. No. 4,874,591. The internal surface area and porosity of sorbents increase drastically during calcination. However, at the higher temperatures, above 1000.degree. C. for example, sintering occurs progressively, and the calcium oxide particles rapidly lose porosity and internal surface area.
Sulfation occurs subsequently to calcination. In other words, CaO reacts with SO.sub.2 and H.sub.2 S gases to form solid sulfate, sulfite or sulfide (CaSO.sub.4, CaSO.sub.3 or CaS). The extent of magnesium oxide reaction with SO.sub.2 is not defined but is known to be much smaller than with calcium oxide. The role of MgO is believed to be in altering the pore structure of the sorbent to one that is more favorable for diffusion of SO.sub.2 to the interior of the particle (Cole et al., Paper 16 Proceedings: 1986 Joint Symposium on Dry SO.sub.2 and Simultaneous SO.sub.2 /NO.sub.x Control Technologies. 1, EPRI CS-4966, December 1986.) Furthermore, it has been reported that the presence of MgO promotes the catalytic oxidation of any existing SO.sub.3 to SO.sub.2. (Flagan, et al., Fundamentals of Air-Pollution Engineering. Prentice-Hall, New Jersey, 1988.) The reactions may potentially occur in the internal pore surface of the CaO particles as well as upon the external particle surface. However, because of the high molar volume of the calcium sulfate (3.3 times that of CaO) the reaction product induces pore filling and entrance closure in the sorbent particle. Hence, the outer layer reacts first to form calcium sulfate, the pores plug up, and the core remains unreacted. Although the sorbent particles may be ground to micron size to minimize this waste, such an adjustment step is prohibitively expensive for power plant applications and other large-scale uses.